Move plugins to manifest, pin Docker version, add Makefile

- Add plugins.txt listing all plugins for reproducible installs
- Add Makefile with setup/start/stop/install-plugins targets
- Remove user/plugins/ from git tracking
- Pin Docker image to 1.7.49.5-ls244

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plugins/admin/themes/grav/templates/partials/dashboard-problems.html.twig

@@ -1,11 +0,0 @@
-{% if authorize(['admin.plugins', 'admin.super']) and grav['flex_objects'] is null %}
-    <div id="dependency-missing" class="dashboard-item-flex">
-        <div class="alert warning">
-            <h1>Admin plugin dependency is missing</h1>
-
-            <p>Admin now requires the Flex Objects plugin for managing <em>User Accounts</em> and <em>Flex Pages</em>. For more information, please <a href="https://learn.getgrav.org/17/admin-panel/faq#flex-objects">check the documentation</a>.</p>
-            <p><a href="{{ admin_route('/plugins/flex-objects') }}" class="button"><i class="fa fa-download"></i> Install and enable Flex Objects plugin</a></p>
-        </div>
-    </div>
-
-{% endif %}